There is an ongoing need in various areas of industry for a sensor system that can be incorporated into a structure in order to measure forces that the structure is subjected to and, ideally, to determine any changes to the structure caused by the application of such forces. It is known to utilise longitudinally arranged optical fibres as strain sensors. When the optical fibres are subjected to strain, the light transmission through them varies, and this variation in light transmission is measured in order to determine the degree of strain. A problem associated with such optical fibre sensors is that the variation in the transmitted light is very small, and thus extremely sensitive detection equipment is required in order to extract meaningful data.